linux-kernel - Re: [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL

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Date:   Tue, 20 Jun 2023 17:22:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        James Houghton <jthoughton@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for
 FOLL_PIN

On 20.06.23 01:10, Peter Xu wrote:
> follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
> target of FOLL_WRITE either.  However add the checks.
> 
> Namely, either the need to CoW due to missing write bit, or proper CoR on

s/CoR/unsharing/

> !AnonExclusive pages over R/O pins to reject the follow page.  That brings
> this function closer to follow_hugetlb_page().
> 
> So we don't care before, and also for now.  But we'll care if we switch
> over slow-gup to use hugetlb_follow_page_mask().  We'll also care when to
> return -EMLINK properly, as that's the gup internal api to mean "we should
> do CoR".  Not really needed for follow page path, though.

"we should unshare".

> 
> When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> clear that it just should never fail.
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>   mm/hugetlb.c | 24 +++++++++++++++---------
>   1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f75f5e78ff0b..9a6918c4250a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6463,13 +6463,6 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   	spinlock_t *ptl;
>   	pte_t *pte, entry;
>   
> -	/*
> -	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> -	 * follow_hugetlb_page().
> -	 */
> -	if (WARN_ON_ONCE(flags & FOLL_PIN))
> -		return NULL;
> -
>   	hugetlb_vma_lock_read(vma);
>   	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
>   	if (!pte)
> @@ -6478,8 +6471,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>   	ptl = huge_pte_lock(h, mm, pte);
>   	entry = huge_ptep_get(pte);
>   	if (pte_present(entry)) {
> -		page = pte_page(entry) +
> -				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> +		page = pte_page(entry);
> +
> +		if (gup_must_unshare(vma, flags, page)) {

All other callers (like follow_page_pte(), including 
__follow_hugetlb_must_fault())

(a) check for write permissions first.

(b) check for gup_must_unshare() only if !pte_write(entry)

I'd vote to keep these checks as similar as possible to the other GUP code.

> +			/* Tell the caller to do Copy-On-Read */

"Tell the caller to unshare".

> +			page = ERR_PTR(-EMLINK);
> +			goto out;
> +		}
> +
> +		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
> +			page = NULL;
> +			goto out;
> +		}


I'm confused about pte_write() vs. huge_pte_write(), and I don't know 
what's right or wrong here.

-- 
Cheers,

David / dhildenb